Modern Polymer Spect..

Quadrature 0
+
3150 3
Wavenumber
Figure 1-9. DIRLD spectra of atactic polystyrene in the phenyl
CH-stretching region. Transition dipoles for IR bands marked
with + are reorienting totally in phase with the applied dynamic
strain, while those with 0 are moving at rates substantially
diffei-ent from the strain.
This surprising discovery reveals that a inajor discrepancy exists between the
classical theory of IR dichroisni and actual experimental observations made for
reorientation dynamics of polymer chains. Figure 1-9, for example, indicates that
the intensity of the quadrature spectrum is close to zero, i.e., signals are in phase
with the applied strain for dichroism peaks marked with (t). If indeed IR dichroism
signals at these wavenumbers reflect the time-dependent orieiitational state of
the entire polymer chain, one would conclude that polymers are reorienting instantaneously
under the dynamic deformation. On the other hand, if the dichroism signals
are measured for other IR bands, for example, at the peaks inarked with (O),
the polymer chain seems to reorient at a rate substantially out of phase with tlie
applied strain. This second statement clearly contradicts the conclusion made as a
result of the previous observation. Thus, for a certain polymer system undergoing a
dynamic deformation, the well-accepted classical view of IR dichroisin (i.e.,
dichroic difference must always be linearly proportional to the average orientation
of polymer chains, regardless of the wavenuinber of IR probe, as described in Eq.
(1-7)) is no longer valid.
The logical explanation for tlie experimentally observed, wavenumber-dependent
behavior of the DIRLD phase angle is that the reorientation rates of individual
traiisitioii dipoles in the system are not the same. For a given macroscopic perturbation
such as dynamic strain, different submolecular constituents (e.g., backbone
segments, side chains, and various functional groups comprising the polymer chain)
may respond at different rates, more or less independently of each other. Consequently,
transition dipoles associated with the molecular vibrations of different
parts of a polymer chain have independent local reorieiitational responses not fully
synchronized to tlie global motioiis of tlie entire polymer chain.